Inherent in conventional hydraulic press design is a lower platen or bed rigidly connected to a stationary upper platen by posts or tie rods. An acting or movable platen that is guided by these tie rods is moved upward or downward forcibly by a hydraulic cylinder mounted centrally above the upper stationary platen or below the lower stationary platen with the piston rod connected to the acting or movable platen.
In this traditional press structure hydraulic pressure generated by an associated hydraulic power and control unit acts on the large area side of the piston, i.e. the side having no piston rod in order to move the piston, rod, and platen generally away from the cylinder body. The frame, massive steel plates, tie rods, opposing stationary platen, etc. are necessary to support the entire force developed by the hydraulic pressure acting on the piston. This frame accounts for a great deal of the total weight, bulk and expense associated with the traditional hydraulic press.
Also as the piston forces the movable platen against the opposing stationary platen, there is a large bending moment created in the stationary platen due to the distance from the center of the hydraulic force to the extreme edges of the stationary platen, resulting in the need for the stationary platens to be of a greater thickness. That is to say, the maximum bending moment for the single hydraulic cylinder is the product of the maximum hydraulic force acting at the center of the greatest span of the opposing platen.
Furthermore, with the use of a single hydraulic ram it is not possible to put robotic mechanisms, vacuum supplies, or knock-out rods in or near the center of the stationary (force) platen.
Also, it is standard practice in the design of the hydraulic press to require high approach speed in order to minimize cycle time. Then, when the mold halves are in close proximity, a multitude of possible requirements exist if one considers the complexity of possibilities of general purpose molding.
The basic solution of these on-going problems lies in providing repeatable precise control of tonnage, velocity, and rate of change thereof. However with a conventional press this precise control can only be approached after a major addition of auxiliary components and expense, or in many cases, simply not at all.
In addition, the cracking (early opening) force utilized in the prior art, is always less than the closing force, and inherently reduces opening velocity. Intrinsically, any increase in either opening force or speed correspondingly decreases the other proportionally. That is to say, a gain in both tonnage and velocity at the same time is not possible, and once implemented, the decision is irrevocable.
Multiple actuator presses have also been proposed and attention is drawn to U.S. Pat. Nos. 3,920,364, 4,295,358, 3,000,295, 3,757,680, 4,502,379 and 3,115,089, all of which relate to prior art presses utilizing a plurality of hydraulic actuators. Of these, the following deserve specific comment.
U.S. Pat. Nos. 3,757,680 and 3,920,364 disclose a press in which the platens are joined by a plurality of actuators in a symmetrical arrangement in which there are no reaction pressures against the press itself. The actuators themselves provide guidance for the platens. This arrangement utilizes actuators providing a different tonnage on molding than on the reverse break direction.
U.S. Pat. No. 4,295,358 discloses a press with more or less conventional guides in which a plurality of hydraulic actuators are utilized.
U.S. Pat. No. 3,000,295 discloses a press using a plurality of double acting hydraulic cylinders for its operation. This arrangement is not believed particularly pertinent.
U.S. Pat. No. 3,115,089 relates to a tandem press mounted coaxially so that the pressure induced into either or both of the cylinders will induce downward movement of the platen 14. The arrangement is that one or both cylinders may be operated to provide different operating pressures and it is specified that if the cylinders have different capacities three different pressure ranges are possible without pressure regulating equipment. This patent suggests using one or both of a pair of actuators operating in parallel.
In general, it is the object of this invention to overcome the foregoing and other difficulties of the prior art designs by the provision of a novel new unireaction hydraulic press, particularly to its inherent controllability, versatility, generally simple construction and lower cost, both in cost of materials and in energy to operate.
Another object of this invention is to provide for instantaneous interchange of movement (i.e., from up acting to down acting and visa versa).